As automatic transmissions for changing the speed ratio according to drive conditions of a vehicle, belt-drive continuously variable transmissions (CVTs) that continuously change the speed ratio are used.
CVTs can make an efficient use of the engine output, which contributes to improving the fuel economy and the drive performance. Some CVTs in practical use have a metal belt and a pair of pulleys and continuously changes the speed ratio by changing the effective diameters of the pulleys through hydraulic actuation. In such CVTs, a metal loop belt is wound around an input pulley (primary pulley) provided on the input shaft and an output pulley (secondary pulley) provided on the output shaft.
The input pulley and the output pulley each have a pair of sheaves and the width of the groove between the sheaves can be changed seamlessly. By changing the groove width, the pitch radius at each of the input and output pulleys (i.e., the distance from the center of the pulley to where the metal loop belt makes contact in the groove between the sheaves) changes, whereby the speed ratio, the ratio between the rotation speed of the input shaft and that of the output shaft, changes continuously.
More specifically, an ECU (Electronic Control Unit) for controlling a CVT determines, based on the accelerator operation amount and the vehicle speed, a target engine output required by the driver, and then determines the target rotation speed of the primary pulley so as to achieve the target engine output on an optimum fuel-economy curve. The ECU then shifts the CVT continuously by controlling the hydraulic control circuit of the CVT such that the actual rotation speed of the primary pulley that is detected by a primary pulley rotation speed sensor equals the target rotation speed. On the other hand, an ECU for controlling the engine controls the engine by determining a target engine torque based on the target engine output and the engine speed and controlling the throttle opening degree accordingly. As such, through such control, CVTs can make an efficient use of the engine output, which contributes to improving the fuel economy and the drive performance.
Some control apparatuses for such CVTs have, as well as the stepless shift mode described above, a manual shift mode that enables, like a manual transmission, the driver to manually shift the CVT among a plurality of predetermined speed steps (stepped shift mode, sequential shift mode).
The speed steps for the manual shift mode correspond to predetermined speed ratios of the CVT. In the manual shift mode, the driver can select any of the speed steps regardless of the vehicle speed and the throttle opening degree.
In a vehicle with a CVT having such a manual shift mode, typically, an upshift “+” switch and a downshift “−” switch are provided as selector switches of the shift lever, and in the manual shift mode, the CVT is manually shifted, like a manual transmission, in response to the shift lever being operated, so that the speed ratio changes non-continuously. Such shift control is also called “sequential shift”.
To enable such sequential shift with a CVT, for example, speed ratios (1) to (5) are set in advance to provide five speed steps for the manual shift mode, and the speed ratio (N) of the CVT changes sequentially from one speed ratio to the other among the speed ratios (1) to (5) in response to the upshift “+” switch and the downshift “−” switch being operated via the shift lever. The speed ratios (1) to (5) for the manual shift mode correspond to the gears (1st, 2nd, 3rd, 4th, 5th gears) of a manual transmission.
However, such a manual shift mode of a CVT may cause a problem that the CVT operates in a manner different from that required or expected by the driver. A technology for solving such problem is described in the patent application publication indicated below.
Japanese Patent Application Publication JP-A-2005-140174 describes a control apparatus of a CVT for a vehicle that smoothly switches the shift mode of the CVT from a stepless shift mode to a stepped shift mode without causing uncomfortable feeling of the driver and satisfies the acceleration requests from the driver during the stepped shift mode. This control apparatus controls the speed ratio of the CVT that can continuously changes the speed of rotation output from the internal combustion engine of the vehicle and has a stepless shift mode in which the speed ratio of the CVT continuously change and a stepped shift mode in which the speed ratio of the CVT changes in steps. This control apparatus includes drive condition detecting means for detecting the drive conditions of the vehicle, target speed ratio setting means for setting the target speed ratio in accordance with the detected drive conditions of the vehicle, speed ratio controlling means for controlling the speed ratio of the CVT to the set target speed ratio, shift mode switching means for switching the shift mode between the stepless shift mode and the stepped shift mode, and rotation speed detecting means for detecting the speed of the internal combustion engine. When the shift mode has been switched from the stepless shift mode to the stepped shift mode, the target speed ratio setting means initially uses the speed ratio that was used in the stepless shift mode immediately before the switchover to the stepped shift mode. Then, when the speed of the internal combustion engine reaches a first predetermined value, the target speed ratio setting means changes the target speed ratio to a higher ratio that slows the engine speed down to a second predetermined value that is smaller than the first predetermined value. The drive condition detecting means has throttle opening degree detecting means for detecting the opening degree of the throttle valve of the internal combustion engine. The shift mode switching means prohibits the shift mode to be switched to the stepped shift mode when the opening degree of the throttle valve is decreasing during the stepless shift mode.
According to the CVT control apparatus described above, when the shift mode of the CVT has been switched from the stepless shift mode to the stepped shift mode, the target speed ratio is set to the speed ratio used in the stepless shift mode immediately before the switchover to the stepped shift mode. Therefore, the shift mode of the CVT can be smoothly switched from the stepless shift mode to the stepped shift mode and thus the driver does not feel uncomfortable. Then, if the engine speed increases to the first predetermined value according to the acceleration requests from the driver during the stepped shift mode, the speed ratio of the CVT is then changed to a higher ratio that slows the engine speed down to the second predetermined value that is smaller than the first predetermined value. In this way, as the engine speed increases, the CVT upshifts, so that the engine speed decreases for a moment and then starts increasing again. As such, in the stepped shift mode, the engine speed does not become constant, thereby satisfying the acceleration requests of the driver appropriately. Further, because the shift mode is prohibited to be switched to the stepped shift mode when the opening degree of the throttle valve is decreasing during the stepless shift mode, that is, when the driver does not intend to accelerate the vehicle, the shift mode can be reliably prevented from changing from the stepless shift mode to the stepped shift mode against the driver's intention.
A vehicle has a cruise control system that automatically controls the engine torque and the speed ratio of the automatic transmission so as to maintain the vehicle speed constant without the accelerator pedal being operated by the driver. When the vehicle comes to an uphill slope during cruise control, the speed ratio of the CVT, in order to prevent the vehicle from slowing down, is increased by downshift so that the drive power increases accordingly.
Similar to the above-described manual shift mode, an automatic shift mode is also known, in which a CVT is automatically shifted using, for example, speed ratios (1) to (5) that have been set as five speed steps and based on specific shift conditions defined by, for example, the relation between the vehicle speed and the throttle opening degree and by the relation between the vehicle speed and the engine speed. Hereinafter, such a shift mode will be referred to as “automatic stepped shift mode” to be distinguished from “automatic stepless shift mode” and “manual stepped shift mode”. That is, there are three shift modes for the CVT, “automatic stepless shift mode”, “automatic stepped shift mode”, and “manual stepped shift mode”. In the automatic stepless shift mode, the CVT is continuously shifted. In the automatic stepped shift mode, the CVT is shifted in steps. In the manual stepped shift mode, the CVT is manually shifted (shifted by the driver).
In the automatic stepped shift mode, the speed ratio (N) of the CVT sequentially changes among the speed ratios (1) to (5) in response to an upshift condition coming into effect (like when the upshift “+” switch at the shift lever is operated during the manual stepped shift mode) and in response to a downshift condition coming into effect (like when the downshift “−” switch at the shift lever is operated during the manual stepped shift mode).
In such a CVT, however, after the cruise control switch is turned on during the stepped shift control (i.e., the automatic stepped shift mode or the manual stepped shift ode), the CVT repeatedly upshifts and downshifts among the speed ratios set as the speed steps for stepped shifting as the load on the vehicle changes due to, for example, changes in the grade of the road (e.g., uphill slope) on which the vehicle is traveling. Such repeated shifts are generally called “shift hunting”. The cause of such shift hunting is as follows. In the stepped shift control (i.e., the automatic stepped shift mode and the manual stepped shift mode), the drive power is controlled using the speed steps of the CVT according to the throttle opening degree, and therefore, depending upon the speed ratios set as the speed steps for the stepped shift control, the required drive power changes frequently whereby shift hunting occurs, making the driver feel uncomfortable. However, this issue is not addressed in JP-A-2005-140174.